When selecting a wind turbine for a given operating location, consideration is given to characteristics of the site such as the complexities of the site terrain and the average wind conditions. The turbines chosen can ideally operate at rated power for as much of the time as possible. However, in practice, wind speeds are variable and the turbine must be able to cope with a wide variety of wind speeds. At lower wind speeds the power output will either be zero, if there is negligible wind, or below rated power. Once the wind speed increases to above that required for generation of rated power, the turbine will protect itself from damage, for example, by varying the pitch of the blades to reduce the power extracted from the wind. In extreme cases the turbine may shut down or yaw out of the wind to prevent catastrophic damage. However, an emergency shutdown or yaw procedure takes time and, in some circumstances may not be able to prevent severe damage to turbine components from occurring.
Our pending application GB 1016493 entitled “Over-Rating Control in Wind Turbines and Wind Power Plants” describes how a turbine, or group of turbines, may be over-rated in response to a request for more power. The term ‘over-rating’ entails extraction of power from the turbine at a level that is greater than rated power. Over-rating may be used for a number of reasons, including in response to a demand from the grid operator which may require a sudden injection of power in response to a grid fault. It is also known to over-rate to take advantage of economic conditions, such as the value of generated electricity at a given time.
We have appreciated that it is desirable to over-rate a turbine when conditions permit. It is possible to monitor parameters which can affect the fatigue life of a turbine, such as turbulence, and only over-rate a turbine when the effect on fatigue life is likely to be low. Thus, a turbine may be operated at above rated power if the wind is considered to be coherent with little turbulence.
One factor which has affected the implementation of over-rating is the risk of an extreme event arriving at a turbine that is operating in an over-rated mode. When a turbine is over-rated, operating loads are very high and an extreme event carries a high likelihood of causing serious damage. Extreme events are defined by International Standard IEC 64100-1, 3rd Edition at Section 6.3.2. Extreme events are conditions which may occur very infrequently, for example, once every year or once every few years, but individually, have the potential to cause serious damage to the wind turbine or individual turbine components. IEC 64100-1 defines a number of extreme events including an extreme operating gust, extreme turbulence, extreme shear and extreme change of direction. Wind turbines are designed to be able to withstand these extreme events and are therefore, effectively, over engineered for operation under normal load conditions.
It is therefore desirable to be able to detect extreme events and to prevent a wind turbine from operating in an over-rating mode when an extreme event is likely to occur. The present invention aims to address this requirement.